Membrane Structure and Function

Introduction

The plasma membrane is a fundamental component of all cells, responsible for regulating the movement of substances into and out of the cell. Its structure and function are essential for maintaining cellular homeostasis and enabling communication with the environment.

Major Ways the Plasma Membrane Regulates Traffic

Overview of Membrane Transport

• Passive Transport: Movement of small molecules across the membrane without energy input, either by diffusion or with the help of transport proteins.

• Active Transport: Movement of small molecules against their concentration gradient, requiring both energy (usually ATP) and a transport protein.

• Bulk Transport (Exocytosis and Endocytosis): Movement of large molecules (such as proteins and polysaccharides) into or out of the cell via vesicles.

Structure of Cellular Membranes

Amphipathic Nature of Membrane Components

• Amphipathic molecules have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

• Phospholipids are the main amphipathic molecules in membranes, forming a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward.

The Fluid Mosaic Model

• The fluid mosaic model describes the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Membrane proteins and lipids can move laterally within the layer, contributing to membrane fluidity.

Membrane Fluidity

• Membranes are held together mainly by weak hydrophobic interactions.

• Most lipids and some proteins can move sideways; rarely, a lipid may flip-flop across the bilayer.

• Temperature Effects:

  • At lower temperatures, membranes become less fluid and may solidify.

  • Membranes rich in unsaturated fatty acids remain more fluid at lower temperatures than those rich in saturated fatty acids.

  • At higher temperatures, saturated fatty acids help maintain membrane integrity, while unsaturated tails may make the membrane too fluid.

• Cholesterol acts as a fluidity buffer in animal cell membranes:

  • At moderate temperatures, it reduces phospholipid movement, decreasing fluidity.

  • At low temperatures, it prevents tight packing, maintaining fluidity.

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Peripheral proteins: Bound to the membrane surface.

• Integral proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

Functions of Membrane Proteins

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix

Role of Membrane Carbohydrates

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids).

• They function as markers for cell recognition and play a role in immune response.

Selective Permeability of the Membrane

Permeability of the Lipid Bilayer

• Small, hydrophobic (nonpolar) molecules (e.g., O2, CO2) pass through easily.

• Hydrophilic (polar) molecules and ions (e.g., glucose, Na+) pass through slowly or not at all without assistance.

Transport Proteins

• Channel proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

• Carrier proteins: Bind to molecules and change shape to shuttle them across the membrane.

• Transport proteins are specific for the substances they move.

Passive Transport

Diffusion

• Movement of particles from an area of higher concentration to lower concentration (down their concentration gradient).

• No energy input is required.

• At dynamic equilibrium, molecules continue to move but there is no net change in concentration.

Osmosis

• Diffusion of water across a selectively permeable membrane.

• Water moves toward the area of higher solute concentration.

Effects of Tonicity on Cells

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic solution: Higher solute concentration outside the cell; cell loses water and shrivels.

• Hypotonic solution: Lower solute concentration outside the cell; cell gains water and may burst (animal cells) or become turgid (plant cells).

Table: Effects of Tonicity on Animal and Plant Cells

Facilitated Diffusion

• Transport proteins (channels and carriers) speed up passive movement of molecules across the membrane.

• Channel proteins can be gated (open or close in response to stimuli).

• Carrier proteins undergo shape changes to move substances down their concentration gradient.

Active Transport

Mechanism and Importance

• Moves substances against their concentration gradients.

• Requires energy, usually from ATP hydrolysis.

• Maintains differences in solute concentrations essential for cell function.

The Sodium-Potassium Pump (Na+/K+ Pump)

• Maintains high K+ and low Na+ inside animal cells.

• Uses ATP to pump 3 Na+ out and 2 K+ in per cycle.

Membrane Potential and Electrochemical Gradients

• Membrane potential: Voltage across a membrane due to differences in ion distribution.

• Electrochemical gradient combines the chemical gradient (concentration) and electrical gradient (charge).

• Electrogenic pumps (e.g., sodium-potassium pump in animals, proton pump in plants) generate membrane potential.

Co-Transport

• Active transport of one solute indirectly drives transport of another.

• Example: Proton pumps create an H+ gradient used to drive uptake of sucrose in plants.

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

• Vesicles fuse with the plasma membrane to release large molecules outside the cell.

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

• Cell takes in macromolecules by forming vesicles from the plasma membrane.

• Three types:

  • Phagocytosis: "Cell eating"; cell engulfs large particles.

  • Pinocytosis: "Cell drinking"; cell takes in extracellular fluid and dissolved solutes.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

• Example: Uptake of cholesterol via LDL particles.

Key Terms and Definitions

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions.

• Phospholipid bilayer: Double layer of phospholipids forming the core of cell membranes.

• Transport protein: Protein that helps substances cross the membrane.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Tonicity: Ability of a solution to cause a cell to gain or lose water.

• Electrochemical gradient: Combined effect of concentration and electrical gradients on ion movement.

Summary Table: Types of Membrane Transport

Example Application: If a cell is placed in a hypertonic solution, water will leave the cell, causing it to shrink. In a hypotonic solution, water enters the cell, which may cause it to burst (animal cell) or become turgid (plant cell).